The following invention relates to an annealing process for light-emitting phosphor material deposited upon a glass substrate for use in thin-film electroluminescent (TFEL) panels.
TFEL panels are flat panel displays which are constructed using a thin layer of electroluminescent (EL) material sandwiched between a pair of insulators, and deposited on a glass substrate forming the face of the panel. The thin-film EL material is excited by an electric field which is supplied by arrays of electrode pairs which sandwich the thin films.
In depositing the EL layer on the glass substrate, it has been known that a heat treatment or annealing step is necessary in order to condition the EL material for optimum light emission. In the past, the annealing has been performed in an oven heated to approximately 450.degree. C. for two hours or more. This creates a bottleneck in the process of manufacturing TFEL panels because of the amount of time required for annealing each panel.
It is well known that light-emitting phosphors are brighter and more efficient if annealed at higher temperatures; however, raising the temperature of the oven has led, in the past, to the warping or cracking of the glass substrate. Moreover, with multicolor displays, using multiple stacked layers or patterned layers of EL material, the problem is aggravated because some EL materials require even higher temperatures to condition the material for greater efficiency. With higher temperature, however, comes the risk of damage to other thin-film layers whose temperature requirements are not as high. Multicolor TFEL displays are shown in copending patent application Ser. No. 727,663, entitled "Multi-Colored Thin-Film Electroluminescent Display" and Ser. No. 844,614, "TFEL Panel Having Multiple Colored Display," both of which are assigned to the same assignee and both of which are incorporated herein by reference.
Another important aspect of the manufacture of TFEL displays is the ease with which such displays can be mass produced. The conventional method of depositing thin-film EL layers is by evaporation which is time consuming and costly. Sputtering is a much more efficient deposition method, but there may be ion damage to the lattice and the grain size of the EL material deposited trough sputtering is usually too small. If the grain size could be made larger, sputtering would be the preferable method of depositing EL layers. The growth of grain size of sputtered EL layers, however, can be accomplished only with the high-temperature, long-time annealing process described above.
Other approaches have included laser annealing, as shown in Johnson, U.S. Pat. No. 4,442,136. With laser annealing, the glass substrate is not warped; however, because of the intense thermal gradient near the boundaries of the laser path, the temperature becomes nonuniform and structural damage may occur to the thin-film layers. This may take the form of delamination of the top insulator layer or the formation of blisters. Moreover, long scan times are required due to the small area covered by the spot size of the laser beam.
Cattell, et al. U.S. Pat. No. 4,552,782 discloses an experimental process for annealing a portion of a DC EL display by heating the substrate to a temperature of greater than 450.degree. C. and then cooling it at a rate of 10.degree. to 20.degree. C. per minute. The problem with this process is that, given the rate of heating and cooling, it is clear that the substrate is brought to the target temperature and then cooled relatively slowly to avoid thermal shock. This leads to problems with glass warpage because the temperature remains too high for too long causing internal stress to the glass.
Vlasenko, U.S. Pat. No. 3,854,070, mentions annealing a panel twice at 600.degree. C. to 700.degree. C. for 5 to 15 minutes each time. How these temperatures are achieved is not specified in the patent. However, the conventional method has been to use an oven with heating elements that heat slowly by both convection and radiation. It takes time to reach these temperatures, and therefore, given the rate of achieving the target temperature in a conventional oven, the glass stays at too high a temperature for too long. At this length of time the panel is likely to warp.
What is needed, therefore, is a process for annealing the thin-film layers of a TFEL screen at a high temperature without warping the glass substrate or structurally damaging the thin-film layers.